1. Field of the Invention
The present invention relates to a semiconductor device having an image sensor function.
2. Description of the Related Art
A semiconductor device having an image sensor function is generally provided with a photoelectric conversion element and one or plural transistors for controlling the photoelectric conversion element.
There are roughly two types of semiconductor devices having an image sensor function: CCD type semiconductor devices and CMOS type semiconductor devices. The CMOS type semiconductor devices are subdivided into those of a passive type with no amplification circuit mounted therein and those of an active type with an amplification circuit mounted therein. An amplification circuit has a function of amplifying an image signal of a subject read by a photoelectric conversion element, making the obtained signal less susceptible to an influence of noise. Accordingly, active CMOS type semiconductor devices provided with amplification circuits find utility in a wide variety of electronic appliances.
A semiconductor device provided with photoelectric conversion elements has a pixel portion as well as a source signal line driver circuit, a gate signal line driver circuit, and a reset signal line driver circuit which are arranged in the periphery of the pixel portion. The source signal line driver circuit includes a bias circuit, a sample hold circuit, a signal output line driver circuit, and a final output amplification circuit. The pixel portion is constituted by x times y pixels arranged in matrix (x and y are natural numbers).
FIG. 11A is a circuit diagram of one pixel 100 arranged in j-th column and j-th row (i and j are natural numbers) of the matrix arrangement. The pixel 100 is arranged within an area defined by one of signal output lines (S1 to Sx), one of power source supply lines (VB1 to VBx), one of gate signal lines (G1 to Gy), and one of reset signal lines (R1 to Ry). The pixel 100 includes an n-channel switching transistor 1120, an n-channel amplification transistor 1130, a p-channel reset transistor 1140, and a photoelectric conversion element 1110. A p-channel-side terminal of the photoelectric conversion element 1110 is connected to a power source reference line 1210.
An explanation of circuit operations is often accompanied by an explanation of transistor operations. An ON-state of a transistor refers to a state where the absolute value of a voltage between the gate and the source of the transistor exceeds that of a threshold voltage for the transistor so that an electrical conduction is established between source and drain regions of the transistor through a channel formation region. On the other hand, an OFF-state of a transistor refers to a state where the absolute value of a voltage between the gate and the source of the transistor is below that of a threshold voltage for the transistor so that no electrical conduction is provided between the source and drain regions of the transistor.
Upon being irradiated with light that is reflected from a subject, the photoelectric conversion element 1110 included in the pixel 100 undergoes a change in its electric potential. More specifically, a potential of an n-channel-side terminal of the photoelectric conversion element 1110 changes. Selecting the gate signal line (Gj) in this state causes the switching transistor 1120 to turn on, whereby the potential of t the n-channel-side terminal of the photoelectric conversion element 1110 is read out in the form of a signal. The signal thus outputted to the signal output line (Sj) is then supplied to the source signal line driver circuit 101.
The term storage time as used herein refers to a period of time from when a photoelectric conversion element arranged in a pixel is initialized until a signal is outputted from the pixel. In other words, it is a period of time during which light is irradiated onto a light receiving portion of the photoelectric conversion element to thereby store the signal to be outputted, and as such it corresponds to a period of time also referred to as exposure time. In addition, the term saturation refers to a state where a potential of the n-channel-side terminal of the photoelectric conversion element 1110 has fallen upon irradiation of extremely bright light and has become equal to a potential of the power source reference line 1210 with no further changes in its value.
An amplitude of a signal inputted to each n-channel transistor is set to Vdd (Hi, H level)−Vss (Lo, L level) regardless of whether the signal is outputted from the reset signal line or the gate signal line. Also, an amplitude of a signal inputted to each p-channel transistor is set to Vss (Hi, H level)−Vdd (Lo, L level) regardless of whether the signal is outputted from the reset signal line or the gate signal line. In the initial state, the respective potentials of the source signal line (Si), the gate signal line (Gj), the reset signal line (Rj), and the power source reference line 1210 are all set to Vss, whereas the potential of the power source supply line (VBi) is set to Vdd.
Next, brief description will be made of connection arrangements for the p-channel reset transistor 1140 as well as how it operates. The source region of the reset transistor 1140 in FIG. 11A is connected to the power source supply line (VBi) and the drain region thereof is connected to the n-channel-side terminal of the photoelectric conversion element 1110. Also, the gate electrode of the reset transistor 1140 is connected to the reset signal line (Rj). Further, in the pixel 100 shown in FIG. 11A, the p-channel-side terminal of the photoelectric conversion element 1110 is connected to the power source line 1210 and the n-channel-side terminal thereof is connected to the source region of the reset transistor 1140.
When the reset signal line (Rj) in the j-th row is selected, a signal of Vss (Hi) potential is inputted to the gate electrode of the p-channel reset transistor 1140. Then, a voltage Vgs between the gate and the source thereof becomes zero or lower, whereby the reset transistor 1140 is turned on. At this time, the potential of the source region of the reset transistor 1140 that is connected to the power source supply line (VBi) is Vdd. Thus, a potential Vpd between the both terminals of the photoelectric conversion element 1110 becomes equal to the potential Vdd of the power source supply line (VBi) (Vpd=Vdd).
Next, description will be made of a relationship between an intensity of light irradiated onto the photoelectric conversion element 1110 and a potential of the photoelectric conversion element 1110, with reference made to FIG. 11B. Referring to FIG. 11B, a solid line indicates the potential Vpd of the photoelectric conversion element 1110 upon irradiation of dark light, a dotted line indicates the potential Vpd of the photoelectric conversion element 1110 upon irradiation of bright light, and a broken line indicates the potential of the reset signal line Rj.
The photoelectric conversion element 1110 stores electric charges generated by light irradiated thereto during storage time. Thus, even when lights having-the same intensity are irradiated, a total amount of charges generated by each light and hence the resulting signal value differ if the storage time is varied. As shown in FIG. 11B, when bright light is irradiated to the photoelectric conversion element 1110, a saturation state is reached with short storage time. On the other hand, when dark light is irradiated to the photoelectric conversion element 1110, longer storage time becomes necessary, but the saturation state is eventually reached nevertheless. That is, the signal to be read out from the photoelectric conversion element 1110 is determined by the product of an intensity of light irradiated thereto and a storage time.
In the pixel 100 shown in FIG. 11A, the reset transistor 1140 is a p-channel transistor and the potential difference Vpd between both electrodes of the photoelectric conversion element 1110 has the same value as the potential Vdd supplied though the power source supply line (VBi), thus making it possible to obtain a sufficient signal amplitude. In other words, the potential of the n-channel-side terminal of the photoelectric conversion element 1110 can be sufficiently raised up to Vdd without causing amplitude attenuation.
Next, description will turn to a case where all the transistors included in the pixel 100 are constituted by n-channel transistors, with reference made to FIG. 12A. Note that a threshold voltage of the n-channel reset transistor 1140 is denoted by a symbol VthN.
A brief explanation will be given with regard to an operation of the n-channel reset transistor 1140 shown in FIG. 12A. When the reset signal line in the j-th row (Rj) is selected, a signal of Vdd (Hi) potential is inputted to the gate electrode of the n-channel reset transistor 1140. At the same time, a potential of the drain region of the reset transistor 1140 which is connected to the power source supply line (VBi) becomes Vdd.
At this time, if the voltage Vgs between the gate and the source of the reset transistor 1140 is larger than VthN, the reset transistor 1140 becomes an ON-state. Conversely, if Vgs is smaller than VthN, then the reset transistor 1140 becomes an OFF-state, so that a voltage supplied through the power source supply line (VBi) does not reach the n-channel-side terminal of the photoelectric conversion element 1110. That is, the potential difference Vpd between both electrodes of the photoelectric conversion element 1110 does not become greater than the value (Vdd−VthN) obtained by subtracting the threshold voltage VthN for the reset transistor 1140 from the potential Vdd of the power source supply line (VBi).
Next, description will be made of a relationship between an intensity of light irradiated onto the photoelectric conversion element 1110 and a potential of the photoelectric conversion element 1110, with reference made to FIG. 12B. As described above, the potential difference Vpd between the both electrodes of the photoelectric conversion element 1110 does not become greater than the value (Vdd−VthN) obtained by subtracting the threshold voltage VthN from the potential Vdd of the power source supply line (VBi). Therefore, the greater the threshold voltage VthN, the greater becomes the attenuation of amplitude, so that a sufficient signal amplitude cannot be attained with respect to the potential difference Vpd between the both terminals of the photoelectric conversion element 1110. That is, the greater the)threshold value VthN becomes, the more difficult it becomes to sufficiently raise the potential of the n-channel-side terminal of the photoelectric conversion element 1110. As a result, changes in the potential of the photoelectric conversion element 1110 become so minuscule that there will be little noticeable difference among signals outputted from the pixel 100. In such a case, it becomes difficult to read information of a subject with precision.
Next, description will turn to a case where all the transistors included in the pixel 100 are constituted by p-channel transistors, with reference made to FIG. 14A. Note that a threshold voltage of the p-channel reset transistor 1140 is denoted by a symbol VthP. In the pixel 100 shown in FIG. 14A, the n-channel-side terminal of the photoelectric conversion element 1110 is connected to the power source line 1210, and the p-channel-side terminal thereof is connected to the source region of the reset transistor 1140.
In the arrangement shown in FIG. 14A, when a signal of Vss (Hi) potential is inputted to the reset transistor 1140, the reset transistor 1140 becomes an ON-state. At this time, the potential of the drain region of the reset transistor 1140 is Vss, while the potential of the source region thereof becomes equal to the value (Vss+|VthP|) obtained by adding together the potential Vss of the power source supply line (VBi) and a threshold voltage thereof. Accordingly, it follows that the potential difference Vpd between the both terminals of the photoelectric conversion element 1110 does not become greater than the value obtained by subtracting the sum (Vss+|VthP|) of the potential Vss of the power source supply line (VBi) and the threshold voltage from the potential Vdd of the power source supply line (VBi). In other words, the potential of the photoelectric conversion element 1110 does not become greater than the value of Vdd−(Vss+|VthP|).
Summarizing the foregoing description, the pixels respectively shown in FIGS. 11A, 12A, and 14A each include: three transistors consisting of the switching transistor 1120, the amplification transistor 1130, and the reset transistor 1140; and the photoelectric conversion element 1110. Thus, the three pixels are identical in configuration. However, conductivity types of the transistors differ among the three pixels, as is manifested in the fact that the reset transistor 1140 is a p-channel transistor in FIGS. 11A and 14A, whereas it is an n-channel transistor in FIG. 12A
As described hereinabove, in the pixel shown in FIG. 11A the reset transistor 1140 is a p-channel transistor and the potential difference Vpd between the both electrodes of the photoelectric conversion element 1110 can be sufficiently raised to the power source potential Vdd. On the other hand, in the pixel shown in FIG. 12A the reset transistor 1140 is an n-channel transistor and the potential Vpd between the both terminals of the photoelectric conversion element 1110 experiences amplitude attenuation whereby it does not become greater than the value (Vdd−VthN) obtained by subtracting the threshold voltage VthN from the power source potential Vdd. Also, in the pixel shown in FIG. 14A the reset transistor 1140 is a p-channel transistor and the potential difference between the both terminals of the photoelectric conversion element similarly experiences amplitude attenuation whereby it does not become greater than the value of Vdd−(Vss+|VthP|).
In a semiconductor device, semiconductor elements such as transistors are typically manufactured on an insulating surface or a semiconductor substrate. The resulting complexity of its manufacture has been the source of reduced yield and increased manufacturing costs. Accordingly, utmost simplification of the manufacturing process is a primary object in achieving increased yield and reduced costs. In view of this, the present inventor has conceived of using transistors having a single polarity (i.e. having the same conductivity type) for the pixel portion and for the peripheral driver circuits (the source signal line driver circuit, the gate signal line driver circuit, and the like).
Incidentally, in the pixel 100 shown in FIG. 12A, all the transistors are constituted by n-channel transistors. Thus, the pixel 100 is constituted by transistors having a single polarity. Likewise, all the transistors included in the pixel 100 shown in FIG. 14A are p-channel transistors, and thus the pixel 100 is constituted by transistors having a single polarity. However, amplitude attenuation occurs in the above-mentioned pixels, thus making it impossible to attain a sufficient signal amplitude.
In the pixel 100 shown in FIG. 11A, the potential difference Vpd between the both electrodes of the photoelectric conversion element 1110 is raised to the power source potential Vdd in order to attain a sufficient signal amplitude. However, the pixel 100 includes transistors having mutually different conductivity types, which adds complexity to its manufacture.
To conclude, when the pixel is constituted by transistors having a single polarity (i.e. having the same conductivity type) with the conventional pixel configuration, although the number of manufacturing steps can be reduced, it becomes impossible to attain a sufficient signal amplitude.